Jets seen by Rosetta caused by comet’s strange shape

ESA’s Rosetta spacecraft imaged unusual jets of gas and dust spewing from comet 67P/Churyumov-Gerasimenko every morning at sunrise (left). Computer simulations (right) indicate the jets are a result of the comets rugged topography. Image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The European Space Agency’s Rosetta spacecraft spent two years orbiting the Comet 67P/Churyumov-Gerasimenko, using its OSIRIS camera to snap more than 70,000 images of the strange duck-shaped body. In addition to sudden outbursts of gas and dust, researchers noted jets that developed every morning as sunlight warmed ares of frost on the comet’s surface.

“When the Sun rises over a part of the comet, the surface along the terminator almost instantaneously becomes active,” said Xian Shi of the Max Planck Institute for Solar System Research and lead author of a paper in Nature Astronomy. “The jets of gas and dust, which we then observe within the coma, are very reliable: they are found each morning in the same places and in a similar form.”

The early morning activity is the result of frost, which forms overnight, quickly evaporating in sunlight. OSIRIS principal investigator Holger Sierks said such outbursts “can often be traced back to a small area on the surface where suddenly frozen water is exposed, for example due to a landslide.”

“In the case of cometary activity at sunrise, this is different,” he added. “The frost is distributed fairly evenly over the entire surface.”

But it was not immediately clear why jets formed rather than a homogeneous cloud.

A new study shows they are the result of 67P’s odd shape and jagged topography. Analysing images of the Hapi region on the “neck” of the comet that were taken at different angles, the researchers found that frost evaporated very efficiently in areas that were strongly illuminated and that pits and concave depressions concentrated the gas and dust emissions much like a lens.

The team was able to build computer simulations that closely mirrored the early morning jets seen by Rosetta.

“The complex shape of Rosetta’s comet makes many investigations difficult,” said Shi. “But for this study it was a blessing.” She said gas and dust released from a spherical or potato-shaped comet would be more evenly distributed and might not be as prominent in a comet’s coma.

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